When new, offset values of exhaust gas sensors, such as NOx sensors, may be determined in order to calibrate the sensor so that accurate measurements of exhaust gas constituents may be obtained. Over time, however, sensor parameters can drift resulting in measurement error. For example, an offset and/or slope, which are calibration parameters used to define a linear transformation from sensor output to a measured parameter, may drift during sensor operation. Thus, periodic updates of the sensor parameters, including sensor offset, during the lifetime of the sensor may be used, where gasses of known NOx and/or ammonia (NH3) are passed over the sensor to enable the update.
One approach places the sensor in the engine exhaust. U.S. Pat. No. 6,311,480 details a process to zero out the offset of a NOx signal, either during deceleration fuel shut-off or during lean operation at low load. However, opportunities for passing gasses of known constituent concentrations may be limited. One such opportunity mentioned, for example, is during deceleration fuel shut-off (DFSO) when the exhaust gas is comprised substantially of ambient air. If the sensor is downstream of a NOx storage catalyst, however, there is a possibility that NOx may exit the catalyst in the exhaust stream. Further, the sensor may be cross sensitive to other gasses present in the exhaust stream, such as ammonia. As such, it may be difficult to determine an accurate NOx offset even during DFSO, which may lead to an increase in NOx emissions over time. Another opportunity for passing gasses of known constituent concentrations is, for example, during lean and low load engine operation. During such operation, however, there is a risk of break-through and exhaust gas constituent concentrations (such as NOx and NH3) higher than assumed.
The inventors herein have recognized the above problems and have devised an approach to at least partially address them. Thus, a method of operating an engine system having a NOx sensor coupled to an exhaust gas recirculation system of an engine in a motor vehicle is disclosed. The method comprises, during a first operating condition, directing at least some exhaust gas from an exhaust of the engine through the exhaust gas recirculation system and past the NOx sensor to an intake of the engine and, during a second operating condition, directing at least some fresh air through the exhaust gas recirculation system and past the NOx sensor.
In this manner, during the first operating condition, the sensor can more accurately measure operation, such as NOx levels, as the sensor experiences only a portion of the total exhaust flow. Further, during the second condition, an accurate offset, such as a NOx offset of the sensor, may be generated, while the engine may continue to carry out combustion.
In one specific example, a low pressure-EGR system may be configured such that it has a bypass which includes an EGR purge valve, the bypass being coupled at one end to the EGR passage and at its other end to the intake manifold downstream of a compressor of a turbocharger coupled to the engine. During the second operating condition, when the boost pressure provided by the compressor is greater than a current exhaust pressure, the EGR purge valve may be positioned to allow fresh air from the intake manifold to enter the EGR passage. Due to the pressure difference, the air flow direction in the EGR passage may be reversed during the second operating condition and, as a result, an EGR cooler may be purged of buildup. In addition, an offset of the sensor may be generated as fresh air flows from the intake manifold, past the sensor, and into the exhaust manifold.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.